Process of pickling metals



'ili-liiiiii Patented Feb. 12, 1952 UNITED PROCESS OF PICKLING METALS Marion D. Barnes, New York, N. Y., assignor to Sparex Chemical Company, a partnership No Drawing. Application August 9, 1946, Serial No. 689,525

2 Claims. (01. 134-3) This invention relates to a process of pickling metals.

The invention is particularly useful in removing ferric oxide scale from iron, that is pickling iron and steel products and for that reason will be illustrated in detail by description in connection with such use.

In the making of iron and steel products, it is customary to remove the scale by dipping the products in a solution of a pickling acid or acidic material of which sulfuric acid, sodium bisulfate, hydrochloric acid, and phosphoric acid are examples, the pickling material being in the form of a rather dilute aqueous solution. As the scale is dissolved away, the clean metal is exposed.

The scale on iron includes ordinarily ferric oxide, magnetic oxide, and some ferrous oxide. It is commonly understood that the outer layer of the scale contains iron in the ferric state, either as F8203 or F6304. A layer of ferrous oxide occurs below the ferric layer and in direct contact with the metal itself. It is further considered that, in pickling, the pickling acid penetrates through the outer layer of scale including one of the ferric oxides and first dissolves the undercoating layer of ferrous oxide. When this undercoating dissolves, the outer layer of scale sloughs off, the loosening of this outer or ferric scale being promoted also by hydrogen gas generated by attack of the acid upon the metal itself.

The explanation of the failure of the ferric scale itself to dissolve promptly on contact with the pickling acid is the relatively great insolubility of ferric oxide in water. This insolubility restricts the rate at which hydroxyl' ions are made available for being neutralized by the hydrogen ions of the acid. It is only as the OH- ions are neutralized and thus removed, that more ferric ions dissolve without causing the product of the concentrations of the ferric and hydroxyl ions to exceed the very low solubility product for Fe(H)a. Regardless of the exact mechanism involved, it is a fact that the relatively slow rate of solution of the ferric scale in pickling acid delays the speed of pickling and thus extends the time required to produce the desired clean surface on the metal. This means that there is not only loss of time but also a longer period during which the pickling acid acts upon and dissolves away unnecessary proportions of the metal itself. It

has been estimated, for instance, that approximately 60 pounds of iron per ton is dissolved on the average when iron or steel products are pickled.

A further difficulty arises in pickling with the usual process and composition. As the ferrous ion concentration builds up in the pickling liquor, the rate of further pickling decreases. At concentrations of 18% or so of ferrous ion, calculated as ferrous sulfate in the pickling liquor, the rate of pickling becomes too slow in fact to be commercially satisfactory even though there may be left as much as 5% of free sulfuric acid in the bath.

The present invention provides a method of accelerating the solution of the ferric oxide scale Without depending upon the loosening of it by generation of hydrogen accompanying solution of the metal itself and without waiting for the normally more soluble undercoating of ferrous oxide to be dissolved away first. The invention provides also for limiting the concentration of ferrous ion as it appears in solution in the pickling bath. so as to avoid such concentration of it as to have appreciable reversing or slowing down effect upon the pickling process.

I accomplish these results in accordance with th'pi'esent invention by introducing into the pickling acid a reagent that maintains the concentration of ferric ion at an extremely low level at all times and also places a maximum upon the concentration to which the ferrous ion builds up in the pickling bath, the reagent added being oxalic acid.

With ferric ion the oxalic acid forms a complex which makes the concentration of free ferric ion equal to zero for all practical purposes.

Since the product of the concentration of the hydroxyl ion and ferric ion at saturation is a constant under a given set of conditions, then hindrance to the dissolving of ferric hydroxide.

Then as the scale dissolves and ferrous ion begins to build up in the pickling bath, the oxalic acid causes the formation of ferrous oxalate which has only a low solubility in the dilute pickling acid of the kind described herein, the term pickling acid being used to include such acidic pickling material as sodium bisulfate.

As a result of these effects of the oxalic acid in the pickling solution, there is speeding up of the rate of dissolving of the ferric scale, with no corresponding increase in the rate of dissolving of the iron itself in the solution, and therefore a decrease in the amount of actual metal dissolved. There is also a lengthening of life of the pickling bath in that there is no building up of ferrous bath and regenerated to give a fresh supply-of oxalic acid for reuse in the process. This avoids the diificulty previously experienced in disposing of pickling baths that have reached -.the..inactive stage without acidifying the -water.or..-soil into which the acid bath is discharged.

As the inorganic pickling acid in my composition I use one of those that is conventionalfor pickling iron and steel, as. for example, one of those listed above.

The selected pickling acid is used in aboutthe proportion in which the material is commonly employed for pickling. Thuslmayuse toadvantage 5 to Parts lellt srlsnlndroshloria or phosphoric acid or .15 t0 0 pa rt s of nitercake mimeieiaisbaium bisulfate) for .100 part of the pickling solution.

The oxalic acid is .usedby me in the;proportion ofaboiflff to 10 parts and for bestresults within the range 4 to 10 parts for 100 parts .of the solution, anyoxalic acid above the proportion required for saturation, remaining undissolved. The part of the solution not accounted for by the inorganic acidic material and .the

oxalic acid is principally wat -fl all proportions here and elsewhere herein being expressed as parts by weight.

Theremay be added small amountsof ingredients that are usual in pickling baths, to replace a part of the water in the above formula, s. ,for instance, an acid inhibitor to retard therateof dissolving of metal by the pickling bath and also a wetting agent. I use preferably both an inh bitor and a wetting agent, any that .are conventional in pickling. iron being satisfactory.

In any case, the formula contains oxalic acid in proportion notsubstantially 11 16396 i th inorganic acid.

The inorganic acidic mate als may be used separately or mixed with eac othen; I have found no particular advantage. however. in such mixture and preferto use onlysoneofflllorganic acidic materials in order to. simplify 288 eration and fortification problems as v itll as to required .to .maintain the saturationpronor- =tion wanted for good results inpickling. Thus,

in the presence of dissolved niter cakeofooncentration to parts to! lilllipartsof the pickling solution. th mazdmum. amount i xal c :acid that will dissolve isabqutt; 11015 part -for 100 parts ofthe solution. at themost only. 5.12 .111?

half the amount that will dissolvein theabsenee of-the niter cake atv the same. rcomtemperature- The invention will be. .furthfi illus r ted 1- by description-in connection with-the ton ues-see cific examples.

4 Example 1 A pickling solution is prepared by dissolving niter cake in solution until the specific gravity of the solution becomes 1.25. This corresponds to a proportion of dissolved substance of about .28.-,.3Q%. Oxalicacid isthen addeduntil some of it remains undissolved on standing. An aryl sulfonic acid in amount corresponding to 0.1% of the solution and an equal amount of a quaternary ammonium salt are added to the solution, to serve as wetting agent and inhibitor, respectively,

Iron pipe having a coating of ferric scale or rust is immersed in this bath until clean, after which the pipe is removed and rinsed with water to remove adhering sulfate.

When additional sections of the pipe are immersed and the process repeated a large number 20 of times, the iron in solution builds up to the point atwhich ferrous-oxalate precipitates. This precipitation occurs, the bath still remains-active.

This operation is-continued until tests made at intervals show a sharp risein concentrationof iron in the pickling solution or bath. When-this condition is noted, then more oxalic-acid is-shoveled into the bath and the pickling as described continued.

Theferrous oxalate that=collectsat the bottom of -the bath isthen removed. On alarge 1 scale, it is suitably removed continuously lay-draining solution and ferrous oxalate from thebottom-of the-tank, filtering the suspension, and returning the filtrate to the pickling tank.

Example .2

hemro edu eof Example 1 sfo owedz xcep a f r phosph ri o hy ro hlori aci in the p po t on OH to l per s i rloflpart b wei ht-ofth pick in -s l on.is ubstitu edfo th nite ake or Examplel Th .ra o di so ving of e mist onfthee in in con ct hth kl ns ath f xampl for -instance, ,is -about twice as great as the .rate

of is olvin oon i ons hat are comparable except for-the absence of .the-.oXfl1ic -aci d. The result g u fac c ea nd-free rom. obiectionab y pi ed ems It .wil he. undert ood so mt -is etonde .to cover all han e an modifications ofs zotmeles of h nventionh r in sen/fertil purpo e of. llustration wh h o .:-no cons itut departure irom hei ti =9 $lds99 of in- .vention...

ha Lol im ,prisesoout otin heiron. nd s ale hereonwi h a ueou u io f aci i i a onsist n essentially of sodium ,bisulfate and oxalic. acid in cqmtact.with solidoxalic acidinexcess-of the amount soluble in the solution and the contact-of-the scale withcrystalsof the 1atter i xc of th a u t s hlb am se olution until thescale. dissolved. .2. The methodof removing ferricoxidescale omiton whic omp is s contact n :tha-ito an soa e b r o ih.an u Solu ion o i om i l o si nses en iallvsof oxali acid men c di n r ien l to a romib group consisting of sulfuric ,acid, sodium.,bisulfate a h och oricid. maintaimn the con- .t o itbo sc le with hes lution. un i tbassal s dissolved. and. senaratins rom t solutiomtli resulting precipitate of ferrous oxalate, portion of oxalic acid being maintained at ap- REFERENCES CITED The following references are of record in the file of this patent:

Number 6 UNITED STATES PATENTS Name Date Haggett Nov. 19, 1872 Sawyer Dec. 30, 1873 McCarthy Sept. 27, 1887 Saukaitis Aug. 4, 1936 Ryznar Nov. 4, 1941 Bacon June 10, 1947 Hixson July 1, 1947 

1. THE METHOD OF PICKLING IRON WHICH COMPRISES CONTACTING THE IRON AND SCALE THEREON WITH AN AQUEOUS SOLUTION OF ACIDIC MATERIAL CONSISTING ESSENTIALLY OF SODIUM BISULFATE AND OXALIC ACID IN CONTACT OF SODIUM BISULFATE AND OXALIC ACID IN AMOUNT SOLUBLE IN THE SOLUTION AND MAINTAINING THE CONTACT OF THE SCALE WITH CRYSTALS OF THE LATTER IN EXCESS OF THE AMOUNT SOLUBLE IN THE SOLUTION UNTIL THE SCALE IS DISSOLVED. 